gravitational lock

Gravitational lock, also known as captured rotation, is the tying of the orbital
period of a planet or moon to its axial
period through a tidal effect caused by the gravitational pull of the
primary. The gravitational lock experienced by the Moon, for example, explains
why it always keeps the same face directed toward the Earth. Likewise, the
five inner satellites of Jupiter complete
one orbit for every axial rotation. Mercury is locked in a 3:2 resonance with the Sun so that it completes 3 orbits
of the Sun (each lasting 88 days) in the time it takes to spin twice on
its axis (see Mercury, rotation of).
The opportunity for life to evolve on planets around red
dwarf stars may be compromised by this phenomenon. So small is the habitable
zone around a red dwarf that any planets within this zone may be forced
into a 1:1 lock thus preventing the kind of climatic variations, such as
wetting and drying, that may have been conducive to early biological development
on Earth. It may be possible, however, to envisage alternative pathways
leading to life that do not require a day-night cycle.